The acute coronary ischemic syndromes of unstable angina pectoris and acute myocardial infarction are characterized by intracoronary platelet aggregation and release of platelet agonists. Systemically, there is accelerated platelet turnover, platelet hypersensitivity to the proaggregatory agents TXA2, ADP, and epinephrine, and platelet resistance to the antiaggregatory effects of prostacyclin. Since platelet hypersensitivity to agonists and resistance to prostacyclin can contribute to the development of coronary thrombosis, it has been important to determine the mechanisms for these changes. We and others have described abnormalities in plate TXA2, alpha2-adrenergic, and prostacyclin receptors which may explain the functional platelet abnormalities in acute coronary ischemia. The stimuli and mechanisms for altered platelet receptor expression in acute coronary ischemia are unknown. Platelets, unlike nucleated cells, do not synthesize proteins. Instead, regulation of platelet protein expression is felt to occur during the development of platelet precursor megakaryocytes. Since acute coronary ischemia is associated with increased circulating levels of several platelet agonists, we will study the regulatory effects of these platelet agonists on megakaryocyte development and receptor expression. Because it is impossible to isolate large numbers of human bone marrow megakaryocytes, we will use cultured CHRF-288-11 megakaryoblastic leukemia cells. We have established that these cells are an appropriate model for studying megakaryocyte development and responses in preliminary studies. Herein, we propose to: 1) characterize the acute effects of platelet agonists on CHRF-288-11 cell signaling by measuring intracellular calcium transients, phosphatidylinositol hydrolysis, protein kinase C activation, and mitogen release from alpha granules; 2) determine the effects of platelet agonists on CHRF-288-11 proliferation and megakaryocytic differentiation; and 3) characterize the developmental regulation of platelet receptors during megakaryocytic differentiation by measuring receptor expression and receptor-effector coupling through the use of Northern analysis, radioligand binding to receptors, and parallel assessment of receptor-mediated cell signaling. Completion of the proposed studies will lead to a better understanding of the mechanisms for platelet hypersensitivity in acute coronary ischemia syndromes and may suggest specific therapeutic interventions designed to prevent platelet hypersensitivity in these syndromes.